How is thread synchronization implemented, at the assembly language level?

Question

While I\'m familiar with concurrent programming concepts such as mutexes and semaphores, I have never understood how they are implemented at the assembly language level.

Answer 1

The x86 architecture, has long had an instruction called xchg which will exchange the contents of a register with a memory location. xchg has always been atomic.

There has also always been a lock prefix that could be applied to any a single instruction to make that instruction atomic. Before there were multi processor systems, all this really did was to prevent an interrupt from being delivered in the middle of a locked instruction. (xchg was implicitly locked).

This article has some sample code using xchg to implement a spinlock http://en.wikipedia.org/wiki/Spinlock

When multi CPU and later multi Core systems began to be built, more sophisticated systems were needed to insure that lock and xchg would synchronize all of the memory subsystems, including l1 cache on all of the processors. About this time, new research into locking and lockless algorithms showed that atomic CompareAndSet was a more flexible primitive to have, so more modern CPUs have that as an instruction.

Addendum: In comments andras supplied a "dusty old" list of instructions which allow the lock prefix. http://pdos.csail.mit.edu/6.828/2007/readings/i386/LOCK.htm